1. Field of the Invention
The present invention relates to ultrasonic motors and, more particularly, to an ultrasonic motor rotated by the utilization of oscillation or vibration which is generated by an electric-mechanical energy conversion element.
2. Related Art And Prior Art Statement
A prior art example will be described with technical means disclosed in Japanese Patent Laid-Open No. HEI 3-289378 taken as an example.
FIG. 12 of the attached drawings is an exploded perspective view showing an example of a conventional ultrasonic piezoelectric transducer or vibrator.
As shown in FIG. 12, a pair of piezoelectric elements 203 and 204 reversed in polarizing direction each of which is divided into two with respect to a center thereof, are used as an electric-mechanical energy conversion element. These piezoelectric elements 203 and 204 are arranged such that their polarizing boundary lines shift 90.degree. from each other. Further, a piezoelectric element 209 is the same in configuration as the piezoelectric elements 203 and 204, and a polarizing direction is also divided into two. The ultrasonic vibrator is formed such that the ultrasonic vibrator is put between the piezoelectric elements 203, 204 and 209 and electrode plates 205, 210 and 211, and a center of the vibrator is crimped or forcedly bonded and is fixed by a bolt 206 which is a fastening element. Moreover, a metallic cylinder 201 has a portion of a horn thereof at which a rotor 208 is arranged.
Voltages of sine wave type in which their phases are shifted 90.degree. from each other are applied to the piezoelectric elements 203 and 204, respectively, whereby bending vibration is generated on the ultrasonic vibrator. Thus, the rotor 208 is rotated. A vibrating condition is judged by a voltage value and a phase difference which are generated from the piezoelectric element 209. Frequency of driving voltage is modified or changed such that a motor output is brought to an adequate condition.
Furthermore, Japanese Patent Laid-Open No. HEI 4-91668 is used to describe an another prior art example.
An ultrasonic vibrator disclosed in Japanese Patent Laid-Open No. HEI 4-91668 is arranged such that outer diameters of both ends are thick as compared with an outer diameter of a node position of a driving vibration mode. Configuration of the ultrasonic vibrator is formed in such a manner, whereby it is possible to reduce natural frequency or frequency of vibration of the normal mode as compared with an ultrasonic vibrator which is uniform in diameter. Reversely, a modal mass increases remarkably or considerably so that there can be produced large vibration amplitude. Such technical means is used whereby the arrangement is such that efficiency of the ultrasonic motor is improved, and driving is stabilized.
By the way, in the aforesaid prior art example disclosed in Japanese Patent Laid-Open No. HEI 3-289375, thickness polarization is performed in order to generate bending vibration. The piezoelectric elements each of which is divided into two such that the polarizing directions pass through the center are used for driving and for vibration detection. Such piezoelectric elements have the following problems and so on. That is, since the piezoelectric elements must be reversed in polarizing direction, time is taken for polarizing treatment or processing, and cost increases. Further, since displacements are reversed at the boundary line in the polarizing directions, respectively, large stresses are applied respectively to the piezoelectric elements along the boundary lines. Cracking is generated. An output from the ultrasonic vibrator is reduced. Resonance frequency changes.
Moreover, the technical means in which a large diameter position is arranged at the node position of the vibration so that an attempt is made to increase or enlarge vibration amplitude, as disclosed in Japanese Patent Laid-Open No. HEI 4-91668, is means which is used very well. However, generally, it is feared or apprehended to cause large-sizing and heavy-weighting of the ultrasonic vibrator. If an outer periphery of a portion of an added mass increases with respect to an outer peripheral diameter of the piezoelectric element that is an electric-mechanical energy conversion element, the ultrasonic vibrator is large-sized radially. Meanwhile, if an attempt is made to lengthen the portion of the added mass axially, the total length or entire length of the ultrasonic vibrator is large-sized. Accordingly, in a case where the ultrasonic vibrator is used within a restricted or limited space, this causes a large problem.